Tapered roller bearings and gear shaft support devices

ABSTRACT

A tapered roller bearing and an automotive gear shaft support device can ensure a long endurance life even in the state in which debris is mixed. On the surfaces of an outer ring, inner ring, and tapered rollers formed from carburized bearing steel having an oxygen content of 9 ppm or less, carbo-nitrided layers having a carbon content of 0.80 wt % or over, a Rockwell hardness HRC of 58 or over, and a residual austenite content of 25-35 vol % are formed to increase mechanical properties and fatigue characteristics of the parts and to stably maintain the carbo-nitrided layers on the surfaces of the parts to a quality having suitable toughness, thereby markedly improving the endurance life of the tapered roller bearing in a state in which debris is mixed.

This application is a divisional application of application Ser. No.09/448,941, filed Nov. 24, 1999, U.S. Pat. No. 6,328,477.

BACKGROUND OF THE INVENTION

This invention relates to tapered roller bearings and gear shaft supportdevices for vehicles.

Tapered roller bearings are suitable to support radial load, axial loadand combined load. Because of their large load capacity, they are usedto support gear shafts of power transmission devices such asdifferentials and transmissions in automobiles and constructionmachines.

FIG. 1 shows an automotive differential in which a gear shaft issupported by tapered roller bearings which is one of the embodiments ofthe present invention. It basically comprises a drive pinion 4 rotatablysupported in a housing 1 by two tapered roller bearings 2, 3, a ringgear 5 meshing with the drive pinion 4, a differential gear case 7carrying the ring gear 5 and rotatably supported in the housing 1 by apair of tapered roller bearings 6, pinions 8 mounted in the differentialgear case 7, and a pair of side gears 9 meshing with the pinions 8.These members are mounted in the housing 1 in which is sealed gear oil.The gear oil also serves as a lubricating oil for the tapered rollerbearings 2, 3, 6.

FIG. 10 shows one conventional type of tapered roller bearing. Itcomprises an outer ring 52 having a conical raceway 51, an inner ring 56having a conical raceway 53, a large rib surface 54 on thelarge-diameter side of the raceway 53 and a small rib surface 55 on itssmall-diameter side, a plurality of tapered rollers 57 rollably arrangedbetween the raceway 51 of the outer ring 52 and the raceway 53 of theinner ring 56, and a retainer 58 keeping the tapered rollers 57circumferentially spaced a predetermined distance from each other. Thedistance between the large rib surface 54 and the small rib surface 55of the inner ring is designed to be slightly longer than the length ofthe tapered rollers 57.

The tapered rollers 57 are designed to come into line contact with theraceways 51 and 53 of the outer ring 52 and the inner ring 56 with thecone apexes of the tapered rollers 57 and the raceways 51, 53 convergingon a point O on the centerline of the tapered roller bearing. By thisarrangement, the tapered rollers 57 can roll along the raceways 51, 53.

With such a tapered roller bearing, the raceways 51, 53 have differentcone angles, so that the combined force of loads applied to the taperedrollers 57 from the raceways 51, 53 acts in such a direction as to pushthe tapered rollers 57 toward the large rib surface 54 of the inner ring56. Thus, during use of the bearing, the tapered rollers 57 are guidedwith their large end faces 59 pressed against the large rib surface 54,so that the large end faces 59 and the large rib surface 54 are insliding contact with each other.

On the other hand, since the distance between the large rib surface 54and the small rib surface 55 is designed to be slightly longer than thelength of the tapered rollers 57, as shown enlarged in FIG. 11, thesmall rib surface 55 does not contact the small end faces 60 of thetapered rollers 57 such that small clearances exist therebetween. Also,the small rib surface 55 is formed by a surface inclined outwardlyrelative to the small end faces 60 of the tapered rollers 57. In thebearing manufacturing steps, the small rib surface 55 and the small endfaces 60, which are kept out of contact with each other, are notfinished by grinding.

In mounting such a tapered roller bearing in a mounting position, asshown in FIG. 12A, the assembly comprising the inner ring 56, thetapered rollers 57 and the retainer 58 is inserted into the raceway 51of the outer ring 52 from above with the large end faces 59 of thetapered rollers 57 facing up. At this time, since the tapered rollers 57have freedom relative to the inner ring 56 and the retainer 58, theywill not seat in position, and their small end faces 60 are brought intocontact with the small rib surface 55. This is an initial assembledstate in which clearance δ is present between the large end faces 59 andthe large rib surface 54 of the inner ring 56.

Next, the tapered roller bearing in the initial assembled state istemporarily mounted on a mounting position of a mating device. As shownin FIG. 12B, when break-in is carried out at a low speed of about 50-100rpm while applying an axial load Fa to the end face of the inner ring56, the tapered rollers 57 will move a distance equal to the gap δtoward the large rib surface 54, until as shown in FIG. 12C, the largeend faces 59 come into contact with the large rib surface 54 of theinner ring 56, so that they settle at a regular position during use ofthe bearing where a gap δ exists between the small end face 60 and thesmall rib surface 55.

Thereafter, the tapered roller bearing is preloaded axially under apredetermined load. This preloading is carried out to prevent axialmovement of the tapered rollers 57 during use of the bearing, and tostably bring the tapered rollers into line contact with the raceways 51,53 of the outer ring 52 and the inner ring 56. The control of preloadingforce is carried out by measuring the shaft torque, and preloading endswhen the shaft torque reaches a predetermined value.

Since the power transmission device such as a differential has many gearmeshing portions and sliding portions of rotary members, foreign mattersuch as metallic powder produced by wear at these portions can entergear oil sealed in the housing. Such powder will penetrate into taperedroller bearings for supporting gear shafts, which are rotating underhigh load, thus shortening the working life of the tapered rollerbearings.

Also, when such tapered roller bearings are used to support gear shaftsof a differential which rotates at high speed under high load, since thelarge end faces of the tapered rollers are brought into sliding contactwith the large rib surface of the inner ring, torque due to the slidecontact increases. Further, due to frictional heat buildup, thetemperature of the bearing portion will rise, thus lowering theviscosity of gear oil. This may cause shortage of oil film.

Further, in mounting the tapered roller bearing on a mounting portion,if the gap between the large end faces 59 of the tapered rollers 57 andthe large rib surface 54 is large in the initial assembled state shownin FIG. 12A, break-in time tends to be long until the tapered rollers 57settle in their regular positions shown in FIG. 12C. As shown in FIG.11, since the small rib surface 55 of the inner ring 56 is formedinclined outwardly relative to the the small end faces 60 of the taperedrollers 57, variation in the gap between the large end faces 59 and thelarge rib surface 54 in the initial assembled state is large for thefollowing reasons, and the abovementioned break-in time until all thetapered rollers 57 settle in their regular positions tends to becomeeven longer.

Generally, the small end faces of the tapered rollers remain as forgedsurfaces, so that chamfer dimensions and shape are large in variation.Variations in chamfer dimension and shape are present not only betweentapered rollers but in a circumferential direction of one taperedroller. As shown by solid and chain lines in FIG. 11, if the chamferdimension and shape of the small end faces 60 differ from each other,the following will result. In the case of the small end faces 60 shownby solid line, in the initial assembled state, point P1 on the small endface 60 comes into contact with point Q1 on the small rib surface 55, sothat the gap δ when the tapered rollers 57 settle will be δ₁. On theother hand, in the case of the small end face 60 shown by chain line, inthe initial assembled state, point P2 comes into contact with point Q2,so that the gap δ when the tapered rollers 57 settle will be δ₂. Thus,due to differences in chamfer dimension and shape of the small end faces60, the time until each tapered roller 57 settles in position tends tovary, so that longer break-in time is required.

An object of this invention is to ensure a long endurance life for atapered roller bearing and a gear shaft support device for a vehicle.

Another object is to reduce torque loss and heat buildup due tofriction.

A further object is to shorten break-in time.

SUMMARY OF THE INVENTION

According to this invention, there is provided a tapered roller bearingcomprising an outer ring having a conical raceway, an inner ring havinga conical raceway and formed with a large rib surface on the largediameter side of the conical raceway, a plurality of tapered rollersrollably arranged between the raceway of the outer ring and the racewayof the inner ring, and a retainer for keeping the tapered rollerscircumferentially spaced a predetermined distance from each other,characterized in that the outer ring, the inner ring and the taperedrollers are all formed from a steel having an oxygen content of 9 ppm orless, and that a carbo-nitrided layer having a carbon content of 0.80 wt% or more and a Rockwell hardness HRC of 58 or more is formed onsurfaces of the outer ring, the inner ring and the tapered rollers, andthat the retained austenite content of the carbo-nitrided layer is 25 to35 vol %.

The outer ring, inner ring and tapered rollers are formed from a steelhaving an oxygen content of 9 ppm or less in order to minimize anynonmetallic inclusions formed by oxides in the steel, improve themechanical characteristics and fatigue properties, and to sufficientlyensure bearing life under clean lubricating oil. A steel having anoxygen content of 9 ppm or less can be obtained e.g. by a method ofdegassing molten steel.

Carbo-nitrided layers are formed on the surfaces of the outer ring,inner ring and tapered rollers for the following reasons. Retainedaustenite in a carburized layer obtained by normal carburizing has hightoughness and work hardening properties. Thus a proper amount ofretained austenite ensures hardness of the carburized layer andsuppresses initiation and progression of cracks. But it is unstableagainst heat.

In contrast, if these parts are subjected to carbo-nitriding treatmentunder suitable conditions, nitrogen atoms will solid soluted in retainedaustenite, and thus serve to stabilize the retained austenite againstheat and also properly keep the properties of the carbo-nitrided layeragainst a temperature rise due to temperature rise at the bearingportion. In a carbo-nitrided layer obtained by such carbo-nitridingtreatment, a greater compressive residual stress is formed, so that itis also possible to further increase fatigue strength.

The retained austenite content should be set at 25-35 vol % to give thecarbo-nitrided layer proper toughness, and to relieve excessive increasein stress due to biting of debris. If the retained austenite content isless than 25 vol %, toughness would be insufficient. If over 35 vol %,the hardness would be too low, thus resulting in deterioration insurface roughness due to plastic deformation.

The structure of such a carbo-nitrided layer as mentioned above can beformed by the following treatment steps. After heating and holding thepart for a predetermined time period while keeping the carbon potentialat 0.8% or over in a carburizing atmosphere, it is quenched in oil andis subjected to hardening. Thereafter it is heated and held for apredetermined time period in ammonia gas for nitriding. It is alsopossible to employ a method in which nitriding is carried out duringcarburizing. In order to adjust the retained austenite content, sub-zerotreatment or tempering may be carried out.

According to this invention, a carbo-nitrided layer having a carboncontent of 0.80 wt % or over and a Rockwell hardness HRC of 58 or overmay be formed on the surfaces of the outer ring, inner ring and taperedrollers, the retained austenite amount of this carbo-nitrided layerbeing 25 to 35 vol %, and crownings may be formed at both ends of theraceway of the inner ring, the width of the crowning at each end being20% or less of the width of the raceway of the inner ring.

The crowning is formed at each end of the raceway of the inner ring inorder to prevent excessive edge loads from acting on the rollers and theraceway of the inner ring. The width of these crownings should be 20% orless of the width of the raceway of the inner ring because if it exceeds20%, the contact surface pressure at the central portion of the racewaywould be excessive.

By forming a crowning having a moderate curvature on a portion of theraceway of the inner ring except both ends at which the crownings areformed, the surface pressure distribution on the raceway can be mademore uniform.

According to this invention, the small rib surface of the inner ring maybe formed by a surface parallel to the small end faces of the taperedrollers, the value R/R_(BASE) being 0.75 to 0.87, where R is the radiusof curvature of the large end faces of the tapered rollers, and R_(BASE)is the distance from the apex of the cone angle of the tapered rollersto the large rib surface of the inner ring.

The small rib surface of the inner ring is formed by a surface parallelto the small end faces of the tapered rollers for the following reasons.As shown enlarged in FIG. 6B, by forming the small rib surface 34 of theinner ring 35 from a surface parallel to the small end faces 39 of thetapered rollers 36, it is possible to minimize the influence ofvariations in chamfer dimension and shape of the small end faces 39 ofthe tapered rollers 36 against the gap between the large end faces 38 ofthe tapered rollers 36 and the large rib surface 33 of the inner ring 35in the initial assembled state (which is equal to the gap between thesmall end faces 39 of the tapered rollers 36 and the small rib surfaces34 of the inner ring 35 when the tapered rollers 36 have settled inposition). As shown by chain line in FIG. 6B, even if the chamferdimensions and shapes of the small end faces 39 differ, in the initialassembled state, since the mutually parallel small end faces 39 andsmall rib surface 34 are brought into surface contact, the gap betweenthe large end faces 38 and the large rib surface 33 is always constant.Thus it is possible to reduce differences in time required until eachtapered roller settles and thus to shorten the break-in time.

The ratio of the radius of curvature R of the large end faces of thetapered rollers to the distance R_(base) from the apex of the cone angleof the tapered rollers to the large rib surface of the inner ring,R/R_(base) should be set at 0.75 to 0.87 for the following reasons.

FIG. 7 shows the results of calculation using the Karna's equation,where t is the thickness of oil film formed between the large ribsurface of the inner ring and the large end faces of the taperedrollers. The ordinate shows the ratio t/to, which is the ratio to oilfilm thickness to when R/R_(base)=0.76. The oil film thickness t is themaximum when R/R_(base)=0.76, and decreases sharply when R/R_(BASE)exceeds 0.9.

FIG. 8 shows the results of calculation for determining the maximumhertz stress p between the large rib surface of the inner ring and thelarge end faces of the tapered rollers. The ordinate shows, like FIG. 7,the ratio p/po, which is the ratio to maximum hertz stress po whenR/R_(base)=0.76. The maximum hertz stress p monotonously decreases withan increase in R/R_(base).

In order to reduce torque loss and heat buildup due to sliding frictionbetween the large rib surface of the inner ring and the large end facesof the tapered rollers, it is desirable to increase the oil filmthickness t and reduce the maximum hertz stress p. Based on thecalculation results of FIGS. 7 and 8 and the below-mentioned seizureresistance test results, the present inventors determined the suitablerange of R/R_(base) at 0.75-0.87. For conventional tapered rollerbearings, the R/R_(base) value is designed at a range of 0.90-0.97.

By forming the surface roughness Ra of the large rib surface of theinner ring in the range of 0.05-0.20 μm, the oil film thickness tbetween the large rib surface of inner ring and the large end faces ofthe tapered rollers, and the lubricating condition between thesesurfaces can be maintained in a proper state.

The surface roughness Ra should be 0.05 μm or over for the followingreasons. As shown in FIG. 12B, when the tapered roller bearing ismounted, break-in is carried out at a low speed of 50-100 rpm whileapplying an axial load Fa to the end face of the inner ring 56. If thesurface roughness Ra is less than 0.05 μm, the lubricating state betweenthe large rib surface 54 of the inner ring 56 and the large end faces 59of the tapered rollers 57 will involve a mixture of fluid lubricationand boundary lubrication during break-in, so that the frictioncoefficient varies considerably and the measured shaft torque varieswidely. This worsens the preload control accuracy. If Ra is 0.05 μm orover, the lubricating state will be boundary lubrication, so that thefriction coefficient stabilizes and thus preload control is possiblewith high accuracy. Under normal bearing use conditions where speedexceeds 100 rpm, sufficient oil film is formed between the large ribsurface 54 and the large end faces 59, so that the lubricating statebetween these surfaces becomes fluid lubrication, and the frictioncoefficient decreases.

The surface roughness Ra should be 0.20 μm or under because if Ra isover 0.20 μm, the temperature will rise at the bearing portion in thehigh-speed rotation region, so that when the viscosity of lubricatingoil decreases, the oil film thickness tends to be insufficient andseizure tends to occur.

By restricting the gap δ formed between the small rib surface of theinner ring and the small end faces of the tapered rollers when the largeend faces of the tapered rollers are in contact with the large ribsurface of the inner ring to not more than 0.4 mm, it is possible toreduce the number of revolutions required for the tapered rollers tosettle in position during the break-in, and to shorten the break-intime. The permissible maximum value of the gap δ, that is, 0.4 mm, wasdetermined based on the results of the below-described break-in test.

By forming the small rib surface of the inner ring by grinding orturning, it is possible to accurately control the gap between the smallrib surface of the inner ring and the small end faces of the taperedrollers.

The tapered roller bearing of this invention may have the large ribsurface of the inner ring made up of a conical surface in contact withthe large end faces of the tapered rollers, and a flank smoothlyconnecting with the conical surface and curving in a direction away fromthe large end faces of the tapered rollers.

By smoothly connecting the curved flank to the conical surface of thelarge rib surface of inner ring in contact with the large end faces ofthe tapered rollers and forming an acute-angle, wedge-shaped gap nearthe outer edge of the contact region, it is possible to increase thefunction of drawing lubricating oil into the contact region and to forma good oil film. Also, by the formation of the smooth flank, it ispossible to prevent damage due to abutment with the large rib surface ofinner ring when the tapered roller skews.

By employing an arc as the sectional shape of the flank, it is possibleto easily form a flank that is superior in the lubricating oil drawingfunction.

By providing a circular recess on the central portion of the large endfaces of the tapered rollers, and extending the outer peripheral end ofthe recess to near the boundary between the conical surface and theflank of the large rib surface of the inner ring, it is possible toguide lubricating oil to near the wedge-shaped gap and to supply asufficient amount of lubricating oil into the wedge-shaped gap, and alsoto further increase the permissible skew angle of the tapered rollers.

By providing the boundary between the conical surface and the flank ofthe large rib surface of inner ring near the outer edge of the maximumcontact oval produced by the contact between the large end faces of thetapered rollers and the large rib surface of the inner ring under themaximum permissible axial load of the tapered roller bearing, it ispossible to suitably form the wedge-shaped gap for drawing thelubricating oil in the entire load range of the tapered roller bearing.

Also, in this invention, in a gear shaft support device for a vehicle inwhich a gear shaft is rotatably supported by a tapered roller bearing ina housing in which is sealed gear oil, the outer ring, inner ring andtapered rollers of the tapered roller bearings are formed from a steelhaving an oxygen content of 9 ppm or less, and a carbo-nitrided layerhaving a carbon content of 0.80 wt % or more and a Rockwell hardness HRCof 58 or more is formed on each of their surfaces, the carbo-nitridedlayer having a retained austenite amount of 25 to 35 vol %. Thus it ispossible to markedly prolong the maintenance cycle of differentials andtransmissions, etc.

Other features and objects of the present invention will become apparentfrom the following description made with reference to the accompanyingdrawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical sectional view of a differential in which isassembled a gear shaft support device of a first embodiment;

FIG. 2A is a vertical sectional view of a tapered roller bearing of afirst embodiment;

FIG. 2B is a partially enlarged sectional view of the same;

FIG. 3 is a partially enlarged sectional view of a tapered rollerbearing of a second embodiment;

FIG. 4A is a vertical sectional view of a tapered roller bearing of athird embodiment;

FIG. 4B is a partially enlarged sectional view of the same;

FIG. 5 is a sectional view explaining the design specifications of thetapered roller bearing of FIG. 4;

FIG. 6A is a vertical sectional view of a tapered roller bearing of afourth embodiment;

FIG. 6B is a partially enlarged sectional view of the same;

FIG. 7 is a graph showing the relation between the radius of curvatureof the large end face of the tapered roller and an oil film thickness;

FIG. 8 is a graph showing the relation between the radius of curvatureof the large end face of the tapered roller and a maximum hertz stress;

FIG. 9 is a partially enlarged sectional view of a tapered rollerbearing of a fifth embodiment;

FIG. 10 is a partially omitted vertical sectional view of a conventionaltapered roller bearing;

FIG. 11 is a partially enlarged sectional view of FIG. 10; and

FIGS. 12A-12C are sectional views showing how the tapered roller bearingis mounted.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIGS. 1 and 9, embodiments of this invention aredescribed. FIG. 1 shows, as described above, a differential of anautomobile, in which for the support of the drive pinion 4 and thedifferential gear case 7 on which is mounted the ring gear 5, the gearshaft support device using the tapered roller bearings 2, 3, 6 of theembodiments is adopted.

FIG. 2A shows a tapered roller bearing 6 as a typical example. It has anouter ring 11 having a conical raceway 10, an inner ring 15 having aconical raceway 12, a large rib surface 13 on the large-diameter side ofthe raceway 12, and a small rib surface 14 on its small-diameter side, aplurality of tapered rollers 16 rollably arranged between the respectiveraceways 10, 12 of the outer ring 11 and the inner ring 15, and aretainer 17 for retaining the tapered rollers 16 at predeterminedcircumferential intervals.

The outer ring 11, inner ring 15 and tapered rollers 16 are all formedfrom carburized bearing steel (SCr 435) having an oxygen content of 9ppm or less, and as shown in FIG. 2B, carbo-nitrided layers 11 a, 15 a,16 a having a carbon content of 0.80 wt % or more and a Rockwellhardness HRC of 58 or more, and the retained austenite content of 25 to35 vol % are formed on the surfaces of these parts 11, 15 and 16. Thoughnot shown, the tapered roller bearings 2, 3 have the same structure.

Hereinbelow, the Examples of the first embodiment and its ComparativeExamples are described.

EXAMPLES

Tapered roller bearings (Examples 11-15 in Table 1) in which acarbo-nitrided layer having a carbon content of 0.80 wt % or more and aRockwell hardness HRC of 58 or more and the retained austenite contentof 25-35 vol % was formed on each of the outer ring, inner ring andtapered rollers formed from carburized bearing steel (SCr435) having anoxygen content of 9 ppm or less were prepared. The bearing dimensionswere all 40 mm in inner diameter and 68 mm in outer diameter.

Comparative Examples

Tapered roller bearings (Comparative Examples 11-15 in Table 1) inwhich, similar to the Examples, a carbo-nitrided layer having a carboncontent of 0.80 wt % or over and a Rockwell hardness HRC of 58 or overand the retained austenite content of 25-35 vol % was formed on each ofthe outer ring, inner ring and tapered rollers formed from carburizedbearing steel (SCr435) having an oxygen content exceeding 9 ppm, andtapered roller bearings (Comparative Examples 16, 17 in Table 1) inwhich the outer ring, inner ring and tapered rollers were formed fromcarburized bearing steel (SCr435) having an oxygen content of 9 ppm orless but the carbo-nitrided layer formed thereon had a retainedaustenite content outside the range as claimed in the present inventionwere prepared. Also, a tapered roller bearing (Comparative Example 18 inTable 1) in which carburized bearing steel (SCr435) having an oxygencontent exceeding 9 ppm was used and heat treatment with only ordinarycarburizing was prepared. The dimensions of each bearing were the sameas in Examples of the invention.

A debris contamination life test in which the tapered roller bearings ofthe Examples of the invention and Comparative Examples were mounted on arotary shaft arranged in a case in which was sealed a lubricating oil inwhich was mixed debris, and a clean oil life test in which they weremounted on a rotary shaft arranged in a case in which clean lubricatingoil was circulated were conducted.

The test conditions are as shown below.

(Debris Contamination Life Test)

Load: 11.76 kN

Revolutional speed: 1500 rpm

Lubricating oil: turbine oil VG56 (oil bath)

Debris: gas atomized metallic powder (particle diameter: 100-180 μm,hardness: HV 700-800, mixed amount: 1 g/liter)

(Clean Oil Life Test)

Load: 21.56 kN

Revolutional speed: 2000 rpm

Lubricating oil: turbine oil VG 56 (circulation oil supply)

The test results are shown in Table 1. In the debris contamination lifetest and the clean oil life test, the lives were evaluated in terms ofL10 life (time period during which 90% of the bearings were notdestroyed and usable). Also, for the life ratios, the endurance life ofthe bearing of Comparative Example 18, which was manufactured underordinary conditions both in material and heat treatment, was used as areference value.

It is apparent that the tapered roller bearings of the Examples showexcellent results both in the debris contamination life test and cleanoil life test. On the other hand, Comparative Examples 11-15, in whichthe retained austenite content was in the range of 25-35 vol % but theoxygen content of the steel was high, showed good results in the debriscontamination life test, but inferior results in the clean oil lifetest. Also, for Comparative Examples 16-17, in which the retainedaustenite content was out of the range of the present application, theendurance life in the clean oil life test was a relatively high value,but that of the debris contamination life test was inferior.

FIG. 3 shows in enlarged scale a portion of the tapered roller bearingof the second embodiment. It has edge crownings 20 having a width Wcwhich is 20% or less of the width W of the raceway 19, at both ends ofthe raceway 19 of the inner ring 18. At the central portion betweenthese respective crownings 20, a center crowning 21 having an extremelymoderate curvature is formed. The drop amount D of the crownings 20 is20 μm, and outside the crownings 20, recesses 22 are provided.

This tapered roller bearing, too, is used to support a differential gearcase 7 like the one shown in FIG. 1, and each part is formed fromcarburized bearing steel (SCr 435), and like the tapered roller bearing6 shown in FIG. 2, carbo-nitrided layers having a carbon content of 0.8wt % or over and a Rockwell hardness HRC of 58 or over, and the retainedaustenite content of 25-35 vol %, are formed on their surfaces.

Hereinbelow, the Examples of the second embodiment and its ComparativeExamples are described.

EXAMPLES

Tapered roller bearings (Examples 21-25 in Table 2) in which acarbo-nitrided layer having a carbon content of 0.80 wt % or over, aRockwell hardness HRC of 58 or over and a retained austenite content of25-35 vol % was formed on each of the outer ring, inner ring and taperedrollers formed from carburized bearing steel (SCr435), and edgecrownings having a width Wc which was 20% or less of the width W of theinner ring raceway were formed at both ends of the raceway wereprepared. The tapered roller bearings of Examples 21 through 23 wereformed with a center crowning having a crowning amount C of 2 μm at thecenter of the inner ring raceway, while the tapered roller bearings ofExamples 24 and 25 were not. The bearing dimensions are the same as inthe first embodiment.

Comparative Examples

Tapered roller bearings (Comparative Examples 21-24 in Table 2) inwhich, similar to the Examples, a carbo-nitrided layer having a carboncontent of 0.80 wt % or over and a Rockwell hardness HRC of 58 or overwas formed on each of the outer ring, inner ring and tapered rollersformed from carburized bearing steel (SCr435), but the retainedaustenite content in the carbo-nitrided layers was out of the range asclaimed in the present application, and tapered roller bearings(Comparative Examples 25-27 in Table 2) in which the retained austenitecontent was within the range of the present application, but the widthof edge crownings exceeded the range of the present application, or fullcrowning was formed over the entire width of the inner ring raceway wereprepared. In Comparative Examples 22 and 24, the width of edge crowningalso exceeded the range of the present application. Also, a taperedroller bearing (Comparative Example 28 in Table 2) in which the retainedaustenite content and the width of edge crowning were within the rangeof the present application, and the heat treatment was ordinarycarburizing hardening was prepared. Dimensions of each bearing were thesame as in the Examples.

For the tapered roller bearings of the Examples and ComparativeExamples, a debris contamination life test was conducted. The testconditions were the same as those in the first embodiment, and theendurance life was evaluated in terms of L10 life.

The test results are shown in Table 2. For the life ratios in the table,the endurance life of Comparative Example 28, in which the heattreatment was only carburizing hardening, was used as a reference value.For any of the articles so indicated in the Table, seizure occurred atthe central portion of the raceway.

For each of the tapered roller bearings of the Examples, the life ratiowas more than four-fold and showed an excellent endurance life. Also, noseizure occurred at the central portion of the raceway. On the otherhand, Comparative Examples 21-24, in which the retained austenitecontent was out of the range of the present application, had only abouthalf the life ratio of the tapered roller bearings of the Examples. ForComparative Examples 22 and 24, which were large in crowning width,seizure occurred at the central portion of the raceway. Also, forComparative Examples 25 and 26, in which the retained austenite contentwas in the range of the present application, but the crowning width waslarge, the life ratio was good, but seizure occurred at the central partof the raceway. For Comparative Example 27, which was extremely small indrop amount D, peeling occurred at the ends of the raceway, and the liferatio improved little.

FIGS. 4A and 4B show the third embodiment. This tapered roller bearingwas also used for the support of a differential gear case 7 like the oneshown in FIG. 1, and their parts, that is, the outer ring 23, inner ring24 and tapered rollers 25, were formed from carburized bearing steel(SCr435), and carbo-nitrided layers 23 a, 24 a, 25 a having a carboncontent of 0.80 wt % or over and a Rockwell hardness HRC of 58 or overwere formed on the surfaces of these parts as shown in FIG. 4B.

As shown in FIG. 5, the cone angle apex of the tapered rollers 25, andthe respective raceways 26, 27 of the outer ring 23 and inner ring 24converge at one point on the centerline of the tapered roller bearing,and it is manufactured such that the ratio of the radius of curvature Rof the large end faces 28 of the tapered rollers 25 to the distanceR_(base) from point O to the large rib surface 29 of the inner ring 24,i.e. R/R_(base) is in the range of 0.75-0.87. Also, the large ribsurface 29 is ground to the surface roughness Ra of 0.12 μm.

Hereinbelow, the Examples of the third embodiment and its ComparativeExamples are described.

EXAMPLES

Tapered bearings (Examples 31-34 in Table 3) shown in FIGS. 4A, 4B and5, were prepared in which a carbo-nitrided layer having a carbon contentof 0.8 wt % or over and a Rockwell hardness HRC of 58 or over was formedon the surface of each of the outer ring, inner ring and taperedrollers, which were formed from carburized bearing steel SCr435, inwhich the radius of curvature R of the large end faces of the taperedrollers was in the range of R/R_(base)=0.75 to 0.87, and in which thesurface roughness Ra of the large rib surface of the inner ring was 0.12μm. Dimensions of the bearings were the same as in the first and secondembodiments.

Comparative Examples

Tapered bearings (Comparative Examples 31-33 in Table 3) were preparedin which, like the Examples, a carbo-nitrided layer having a carboncontent of 0.8 wt % or over and a Rockwell hardness HRC of 58 or overwas formed on the surface of each of the outer ring, inner ring andtapered rollers which were formed from carburized bearing steel SCr435,but the R/R_(base) ratio was out of the range of the presentapplication, and a tapered roller bearing (Comparative Example 34 inTable 4) in which the heat treatment was only carburized and hardening,and the R/R_(base) ratio was also out of the range of the presentapplication was prepared. Dimensions of the bearings are the same as inthe Examples.

For the Examples and Comparative Examples, a seizure resistance testusing a rotary tester, and the same debris contamination life test as inthe first and second embodiments were conducted.

The test conditions of the seizure resistance test were as follows.

Load: 19.61 kN

Revolutional speed: 1000-3500 rpm

Lubricating oil: turbine oil VG 56 (oil supply rate: 40mililiters/minute, oil temperature: 40° C.±3° C.)

The test results are shown in Table 3. For the life ratios in the debriscontamination life test, the endurance life (L10 life) of ComparativeExample 34 was used as a reference value. Also, seizure in the seizureresistance test occurred between the large rib surface of the inner ringand the large end faces of the tapered rollers.

For each of the tapered roller bearings of the Examples, the endurancelife was good with the life ratio in the debris contamination life testbeing four or more. Also, it is apparent that the limit revolving speedat which seizure occurred in the seizure resistance test was 2700 rpm orover. On the other hand, for Comparative Examples 31-33, in whichcarbo-nitrided layers were formed, but the R/R_(base) ratio was out ofthe range of the present application, although the life ratio was good,the limit revolving speed for the occurrence of seizure was 2500 rpm orunder, and the possibility that seizure may occur under normal useconditions such as in a differential was high. For Comparative Example33, in which the surface roughness Ra of the large rib surface wasrough, it showed a limit revolving speed that was lower than inComparative Example 32 having the same radius of curvature. ForComparative Example 34, in which heat treatment was ordinarycarburizing, and also R/R_(base) ratio was a conventional value, any ofthe test results were inferior.

In the above embodiments, SCr435 was used as a material for each part,but it is possible to use such bearing steels as SCM420, SCM430, SCM435,SCr420, SCr430, SAE5130, and SAE8620.

FIGS. 6A and 6B show the fourth embodiment. This tapered roller bearingis also used for the support of a differential gear case 7 as in FIG. 1,and comprises an outer ring 31 having a conical raceway 30, an innerring 35 having a conical raceway 32 and provided with a large ribsurface 33 on the large-diameter side of the raceway 32 and a small ribsurface 34 on its small-diameter side, a plurality of tapered rollers 36arranged between the respective raceways 30, 32 of the outer ring 31 andthe inner ring 35, and a retainer 37 retaining the tapered rollers 36 atpredetermined circumferential intervals.

The small rib surface 34 of the inner ring 35, as shown enlarged in FIG.6B, is finished to a ground surface parallel to the small end faces 39of the tapered rollers 36 arranged on the raceway 32. It is in surfacecontact with the small end faces 39 of the tapered rollers 36 in theinitial assembled state shown by one-dot chain line in FIG. 6B, and thegap δ with respect to the small end faces 39 of the tapered rollers 36in the state in which the tapered rollers 36 have settled in position asshown by solid line is in the range of not more than 0.4 mm. The smallrib surface 34 may be finished by turning to reduce the cost.

The cone angle apexes of the tapered rollers 36, and the respectiveraceways 30, 32 of the outer ring 31 and inner ring 35 converge, likethe third embodiment shown in FIG. 5, at one point O on the centerlineof the tapered roller bearing, and it is manufactured such that theratio of the radius of curvature R of the large end faces 38 of thetapered rollers 36 to the distance R_(BASE) from point O to the largerib surface 33 of the inner ring 35, i.e. R/R_(base), is in the range of0.75-0.87. Also, the large rib surface 33 is ground to the surfaceroughness Ra of 0.12 μm.

The Examples of the fourth embodiment and its Comparative Examples aredescribed below.

EXAMPLES

Tapered roller bearings (Examples 41-44 in Table 4) were prepared inwhich the radius of curvature R of the large end faces of the taperedrollers was such that the ratio R/R_(base) was 0.75-0.87, the surfaceroughness Ra of the large rib surface of the inner ring was 0.12 μm, itssmall rib surface was formed as a ground surface parallel to the smallend faces of the tapered rollers, and the gap δ was in the range of notmore than 0.4 mm. Bearing dimensions were the same as in each of theabovementioned embodiments.

Comparative Examples

Tapered roller bearings (Comparative Examples 41-43 in Table 4) wereprepared in which the R/R_(base) value was out of the range of thepresent application, the small rib surface of the inner ring wasinclined outwardly relative to the small end faces of the taperedrollers, and the gap δ exceeded 0.4 mm.

For the tapered roller bearings of the Examples of and ComparativeExamples, a seizure resistance test was conducted under the sameconditions as in the third embodiment. Also, for the tapered rollerbearings of Example 42 and Comparative Example 42, a break-in test wasalso conducted. Sample numbers for the break-in test were 66 for Example42 and 10 for Comparative Example 42.

The results of the test are shown in Table 4. Seizure in the seizureresistance test occurred between the large rib surface of the inner ringand the large end faces of the tapered rollers.

For any of the tapered roller bearings of the Examples, the limitrevolving speed in the seizure resistance test was 2700 rpm or over.This shows that the frictional resistance between the large rib surfaceof the inner ring and the large end faces of the tapered rollers issmall. On the other hand, for the tapered roller bearings of theComparative Examples, the seizure occurrence limit revolving speed was2500 rpm or under, and a problem may arise under normal use conditionssuch as in a differential. For Comparative Example 43, in which thesurface roughness Ra of the large rib surface was rough, it showed alower seizure occurrence limit revolving speed than in ComparativeExample 42 having the same radius of curvature R.

For the break-in test results, in the Comparative Examples, the averagevalue of the number of revolutions until the tapered rollers settled inposition was six, whereas in the Examples, this value was about half,i.e. 2.96. In the Examples of the invention, the standard deviation ofvariation in the number of revolutions was also small. Thus, this showsthat it is possible to stably shorten the break-in time.

FIG. 9 shows a portion of the tapered roller bearing of the fifthembodiment. This tapered roller bearing was also used for the support ofa differential gear case 7 as shown in FIG. 1. The large rib surface 41of the inner ring 40 comprises a conical surface 41 a, and a flank 41 bsmoothly connecting with the conical surface 41 and having an arcuatesection, and a chamfer 41 c connecting with the flank 41 b. The conicalsurface 41 a is, like the tapered roller bearing shown in FIG. 5, formedwith point O as its center. The end faces 43 of the tapered rollers 42are each formed as a spherical surface 43 a having a radius of curvatureR that is smaller than the distance Ro from point O to the large ribsurface 41 of the inner ring 40. A recess 44 of a circular shape isformed at the center of the spherical surface 43 a. The outer peripheralend of the recess 44 extends to near the boundary between the conicalsurface 41 a and the flank 41 b of the large rib surface 41.

As mentioned above, during use of the bearing, the tapered rollers 42roll with their large end faces 43 pressed against the large rib surface41, and the spherical surface 43 a is partially brought into contactwith the conical surface 41 a, so that a contact oval 45 is producedbetween these two curved surfaces. The boundary between the flank 41 band the conical surface 41 a is provided near the outer edge of thecontact oval 45, and an acute wedge-shaped gap is defined by the flank41 b and the spherical surface 43 a at a position near the contact oval45.

The contact oval 45 grows larger as the axial load during use of thebearing increases. With this tapered roller bearing, assuming themaximum contact oval under the permissible maximum axial load, theboundary between the flank 41 b and the conical surface 41 a is designedto be near the outer edge of the maximum contact oval, so that thewedge-shaped gap for drawing the lubricating oil will be formed over theentire load range.

The present invention is applicable to various types of tapered rollerbearings.

As described above, for the tapered roller bearing of this invention,each of its parts, i.e. outer ring, inner ring and tapered rollers areformed from steel having an oxygen content of 9 ppm or less, and acarbo-nitrided layer having a carbon content not less than 0.80 wt % anda Rockwell hardness HRC of 58 or over and the retained austenite contentof 25-35 vol % is formed on the surface of these parts. Thus it ispossible to enhance the mechanical properties and fatigue strength ofthe parts, stably maintain the carbo-nitrided layers on the surfaces ofthe parts to a quality having suitable toughness, and markedly improvethe endurance life in debris contamination conditions.

According to this invention, an edge crowning having a width that is 20%or less of the width of the raceway is formed at both ends of the innerring raceway. This prevents seizure by making the contact surfacepressure at the raceway uniform, maintains the carbo-nitrided layers onthe surfaces of the parts stably to a quality having suitable toughness,and markedly improves the endurance life in debris contaminationconditions.

According to this invention, the radius of curvature R of the large endfaces of the tapered rollers is such that the ratio R/R_(base) will be0.75-0.87, and the small rib surface of the inner ring is formed into asurface parallel to the small end faces of the tapered rollers toprevent seizure by reducing torque loss and heat buildup due to slidingfriction between the large rib surface of the inner ring and the endfaces of the tapered rollers, and to shorten the break-in time toimprove efficiency of mounting of the bearing.

Further, according to this invention, a curved flank is smoothlyconnected to the conical surface of the large rib surface of the innerring in contact with the large end faces of the tapered rollers to forman acute wedge-shaped gap to increase the lubricating oil drawingfunction into this contact region, prevent seizure by reducing torqueloss and heat buildup due to the sliding friction, and prevent seizuredue to abutment with the large rib surface of the inner ring duringtapered roller skew.

With the gear shaft support device of this invention, since its gearshaft is supported by the tapered roller bearing of this invention,endurance life improves even under use conditions in which foreignmatter mixes into gear oil, so that it is possible to extremely prolongthe maintenance cycle of a power transmission device such as adifferential.

What is claimed is:
 1. A gear shaft support device for a vehicle inwhich a gear shaft is rotatably supported by tapered roller bearings ina housing in which is sealed gear oil, said tapered roller bearings eachhaving an outer ring, an inner ring and tapered rollers, characterizedin that a small rib surface of said inner ring is formed by a surfaceparallel to small end faces of said tapered rollers, and that the ratioR/R_(base) is 0.75 to 0.87, wherein R is the radius of curvature oflarge end faces of said tapered rollers, and R_(base) is the distancefrom the apex of the cone angle of said tapered rollers to a large ribsurface of said inner ring, wherein a gap δ formed between the small ribsurface of said inner ring and the small end faces of said taperedrollers when the large end faces of said tapered rollers are in contactwith the large rib surface of said inner ring is not more than 0.4 mm.2. The gear shaft support device as claimed in claim 1 wherein the smallrib surface of said inner ring is formed by grinding or turning.
 3. Agear shaft support device for a vehicle in which a gear shaft isrotatably supported by tapered roller bearings in a housing in which issealed gear oil, said tapered roller bearings each having an outer ring,an inner ring and tapered rollers, characterized in that a small ribsurface of said inner ring is formed by a surface parallel to small endfaces of said tapered rollers, and that the ratio R/R_(base) is 0.75 to0.87, wherein R is the radius of curvature of large end faces of saidtapered rollers, and R_(base) is the distance from the apex of the coneangle of said tapered rollers to a large rib surface of said inner ring,wherein the small rib surface of said inner ring is formed by grindingor turning.
 4. A tapered roller bearing comprising an outer ring havinga conical raceway, an inner ring having a conical raceway and formedwith a large rib surface on the large diameter side of said conicalraceway and a small rib surface on the small diameter side thereof, aplurality of tapered rollers rollably arranged between said raceway ofsaid outer ring and said raceway of said inner ring, and a retainer forkeeping said rapered rollers circumferentially spaced a predetermineddistance from each other, wherein during use, said tapered rollers areguided with large end faces thereof in contact with the large ribsurface of said inner ring, characterized in that the small rib surfaceof said inner ring is formed by a surface parallel to small end faces ofsaid tapered rollers, and that the ratio R/R_(base) is 0.75 to 0.87,wherein R is the radius of curvature of the large end faces of saidtapered rollers, and R_(base) is the distance from the apex of the coneangle of said tapered rollers to said large rib surface of said innerring, wherein a gap δ formed between the small rib surface of said innerring and the small end faces of said tapered rollers when the large endfaces of said tapered rollers are in contact with the large rib surfaceof said inner ring is not more than 0.4 mm.
 5. The tapered rollerbearing as claimed in claim 4 wherein the small rib surface of saidinner ring is formed by grinding or turning.
 6. A tapered roller bearingcomprising an outer ring having a conical raceway, an inner ring havinga conical raceway and formed with a large rib surface on the largediameter side of said conical raceway and a small rib surface on thesmall diameter side thereof, a plurality of tapered rollers rollablyarranged between said raceway of said outer ring and said raceway ofsaid inner ring, and a retainer for keeping said rapered rollerscircumferentially spaced a predetermined distance from each other,wherein during use, said tapered rollers are guided with large end facesthereof in contact with the large rib surface of said inner ring,characterized in that the small rib surface of said inner ring is formedby a surface parallel to small end faces of said tapered rollers, andthat the ratio R/R_(base) is 0.75 to 0.87, wherein R is the radius ofcurvature of the large end faces of said tapered rollers, and R_(base)is the distance from the apex of the cone angle of said tapered rollersto said large rib surface of said inner ring, wherein the small ribsurface of said inner ring is formed by grinding or turning.